It depends on your distribution. If it comes with stock sysklogd, you can not do that. Some, most notably Fedora, already ship with
rsyslog. With it, it is easy. You can also replace stock sysklogd with rsyslog or run it alongside just to get hold of the PRI. Details in this guide:
http://www.rsyslog.com/doc-rsyslog_recording_pri.htmlHTH